Flame-retarded paper products and method and apparatus for making them

ABSTRACT

A flame-retarded paper product comprises as a fire-retardant (FR) ingredient a compound selected from among inorganic and organic flame-retardant compounds. A method of making a flame-retarded paper product is described, which comprises: a) producing a base paper; b) applying to the paper base at least one FR compound, to form a layer comprising said compound; and c) applying to the base paper, over said layer, a coating color and/or laminate and/or surface sizing.

FIELD OF THE INVENTION

[0001] The present invention relates to flame-retarded paper productsand to processes and methods for making the same. More particularly, theinvention relates to flame-retarded paper products, comprising as anactive ingredient a compound selected from among inorganic and organicflame-retardant compounds, particularly ammonium bromide and itsmixtures with other ammonium salts, e.g., ammonium sulfate, mono- ordiammonium phosphate, ammonium polyphosphate, ammonium sulphate, urea,organic brominated flame retardants and their mixtures. Moreparticularly, it relates to a paper product comprising a paper base, alayer comprising flame-retardant (FR) compound or compounds, and a layerof a coating color and/or laminate and/or surface sizing overlying theFR layer. It also relates to a method and apparatus for making such aproduct.

BACKGROUND OF THE INVENTION

[0002] An unfortunate feature of modern life is the large number offires which occur in occupied buildings. Statistics show that in many ofthese fires paper and packaging materials are the first item ignited.The majority of such fires result from ignition using small energysources such as matches, and many of these fires could have been avoidedif the paper had been treated with a flame-retardant additive designedto impart ignition resistance.

[0003] The major component of paper is wood fibers, mainly containingcellulose. Natural cellulose is a polysaccharide consisting of β-glucoseunits linked by ether bonds. When cellulose is burned, a mixture ofgaseous and solid combustion products result and the burning takes placeboth at and above the surface. A study of the pyrolysis of celluloseshows that the major product is a tarry substance that consists mainlyof laevoglucosan. If the formation of laevoglucosan is prevented, theflammability of cellulose is reduced. This is achieved by the use ofphosphorus containing compounds such as ammonium phosphates whichrelease phosphoric acid when strongly heated, and the resultant acidesterifies the glucose units to produce glucose-6-phosphate. Thephosphoric acid also promotes the formation of a solid carbonaceous charin the cellulose which does not support combustion.

[0004] An important feature of burning paper is after-glow, which occursusually after the flames are extinguished and which could igniteadjacent materials. Glow is defined as the combustion of a materialwithout flame but with emission of visible light from the combustionzone surface. The phenomenon of after-glow is poorly understood but isbelieved to involve the carbon at the surface of the cellulose, carbondioxide, carbon monoxide and oxygen. Phosphorus-based flame-retardantsare effective at preventing after-glow and are assumed to function intwo ways (a) by forming a film of condensed acid over the surface of thecellulose thereby preventing access of oxygen to the burning site (b) byabsorbing some of the heat required for further combustion of thecellulose.

[0005] Burning of cellulose in the gas phase proceeds by a free radicalchain reaction mechanism involving hydroxy, oxygen and other reactivefree radical species. Obviously if these active radicals can beneutralized, combustion will cease. Halogen-based materials,particularly bromo-compounds, are effective in terminating free radicalchain reactions and are widely used as flame-retardant additives for awide variety of substrates including paper. Chlorine compounds have thesame mode of action as bromine compounds. The net effect of the use ofthese materials is the production of the deactivating halogen freeradical.

[0006] Flame retardancy cellulosic materials are disclosed in U.S. Pat.No. 3,667,999, in which a basic cellulose structure is treated byimmersion in a bath containing, inter alia, ammonium bromide. U.S. Pat.No. 4,102,794 teaches impregnation of various materials with an aqueoussolution comprising at least three ions selected from ammonium, zinc,bromine, chlorine ions, alkaline-earth metal ions and phosphate ions.U.S. Pat. No. 4,888,136 teaches impregnation of cellulosic materialswith a composition comprising ammonium bromide and, at least, onewater-soluble aluminum salt of an organic hydroxy acid, such as aluminumcitrate or lactate.

[0007] While it is known in the art to use flame-retardant compositions,including those based on ammonium bromide, to impart flame retardancy topaper products all prior art methods are based on the impregnation ofthe paper product. Impregnation presents several disadvantages: theflame-retardant material is easily leached away from the paper, thusdiminishing the flame-retardant effect, and relatively high amounts offlame-retardant material are needed, which are not only expensive, butwhich adversely affect the properties of the paper product. The paperproperties are changed, the brightness and the tensile strength decreasewith increased amount of flame retardant.

[0008] The flame retardant slowly diffuses under the influence of airand acquires a yellowish color. The accelerated aging causes more rapiddiffusion into paper.

[0009] It is important that additive flame retardants—hereinafter,briefly, FR—do not impair the desirable characteristics and propertiesof the paper, such as strength, flexibility and brightness.

[0010] It is clear that there is a great need for a method andcompositions which permit to reduce the amount of flame-retardantmaterial employed, while maintaining a high level of flame-retardancywith time.

[0011] While it is highly desirable to render the paper productsflame-retarded to a substantial extent, flame-retarding inevitablyresults in some deterioration of mechanical properties, particularlytensile strength and tearing strength. It would be important to reducesaid deterioration to a minor and possibly irrelevant degree and tomaintain the ratio between tensile strength and tearing strengthsubstantially unvaried.

[0012] It is therefore a purpose of this invention to provide paperproducts, particularly in sheet form, that are adequately flame-retardedand have a surface that is coated so as to accept printing, writing anddrawing. The term “paper”, as used herein, is meant to indicate allkinds of paper products including, but not limited to, paper board.

[0013] It is another purpose of this invention to provide flame-retardedpaper products having a combination of a high degree of flame-retardancyand mechanical properties that are substantially the same than, or arevery close to, the properties of the conventionally coated base paper.

[0014] It is a further purpose of this invention to provide a method ofapplying the FR compound or compounds to the base paper, which willresult in a product having industrially acceptable mechanicalproperties, not substantially inferior to those of paper that does notcontain FR compounds, or at least are changed to a very minor andsubstantially irrelevant degree.

[0015] It is a still further purpose of this invention to provide amethod of applying the FR compound or compounds to a base paper whichwill result in a paper product having substantially undiminished, oronly slightly diminished, tearing strength and tensile strength andhaving the same ratio between the two said parameters as the same paperto which the FR compound or compounds have not been applied.

[0016] It is a still further purpose of this invention to provide anapparatus for the production of paper products, in particular a coatingapparatus, that is modified with respect to the conventional apparatusin such a way as to allow to carry out the aforesaid method.

[0017] Other purposes and advantages of this invention will appear asthe description proceeds.

SUMMARY OF THE INVENTION

[0018] This invention relates to flame-retarded paper productscomprising as an active ingredient a compound selected from amongammonium bromide, diammonium phosphate, ammonium sulfate, ammoniumpolyphosphate, urea, brominated organic flame retardants and theirmixtures.

[0019] Throughout the description to follow, ammonium bromide will bereferred to as the flame-retardant but it should be understood that thisdoes not involve a limitation and that the examples and statements madewith reference to it apply, mutatis mutandis, to other flame-retardantsas well.

[0020] More specifically, this invention provides a paper product, whichcomprises a base paper, a layer of an FR compound or compounds,preferably including a first binder, and an uppermost layer of a coatingcolor.

[0021] The first binder is preferably starch. The coating color layeralso preferably comprises a second binder, which is preferably the sameas the said first binder but it could be styrene, acrylate, vinylacetate, etc.

[0022] The invention further comprises a method for making the aforesaidpaper product, which method comprises: a) producing a base paper; b)applying to the base paper at least one FR compound, to form a layercomprising said compound; and c) applying to the paper base, over saidlayer, a coating color.

[0023] The base paper is made by conventional processes and apparatusand need not be particularly described. The FR compound is appliedpreferably by spraying on the base paper a liquid phase comprising theFR compound and a first binder. The first binder preferably starch, butcould also be a latex or a polymeric binder or a combination thereof.Other ways of applying the FR compound, e.g. spreading, may be used, butspraying is preferred, particularly because of the low viscosity of theliquid phase. Preferably, the FR compound layer is dried before applyingthe coating color.

[0024] The coating color also comprises a second binder, which may bethe same as the first binder and is preferably starch, and a pigment orpigments chosen by conventional criteria, and can be applied in anyconvenient way, such as by providing an aqueous bath of the color andbinder and applying the aqueous phase of the color and binder onto thepaper by any convenient coating means, or by spreading said aqueousphase onto said paper. It should be understood that the application ofcoating color to a base paper is conventional and need not beparticularly described. Preferably, the thickness of the coating colorlayer is controlled by a blade or the like.

[0025] Preferably, in the paper product of the invention, after drying,the FR compound layer is located between the base paper and the coatingcolor layer, which has a thickness of about 5 to about 30 μm. Thecoating may consist substantially of calcium carbonate, which may or maynot comprise additional pigments, e.g., colors or whitening agents, andbinders.

[0026] The invention also encompasses a flame-retarded paper productcomprising a flame-retardant or flame-retardant mixture in an amount ofbetween 0.01% by weight of the paper to about 10% by weight of thepaper, on a dry basis.

[0027] The invention also provides an improvement to paper coatingmachines, which comprises adding to said machines, before the coatingapplication components, viz. the coating unit, a spraying deviceprovided with nozzles, and preferably adding dryer means between saidspraying device and said coating unit.

[0028] The remaining components of the coating machine are the same asin conventional operations. The coating machine can be integrated intothe paper machine in a way that is conventional and well known toskilled persons. Such integration therefore will not be described, butshould be understood as optionally comprised in the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The drawing (FIG. 1) schematically illustrates a paper coatingmachine, which comprises all the conventional components of suchmachines and additionally comprises means for spraying a compositioncomprising the flame-retardants and the binder onto the paper base, anddrying said composition.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0030] The present invention will be illustrated by specific exampleswhich are not intended to limit it.

[0031] The drawing illustrates schematically a flame-retarding andcoating apparatus, which can be inserted into or otherwise coupled to apaper making machine in any convenient way, easily devised by skilledpersons to suit the specific paper making machine.

[0032] The base paper is a continuous sheet 10 supplied from a drum 11.It travels from right to left looking at the drawing. Numeral 12indicates guide rollers which bring sheet 10 to a position in which anFR solution 13 is sprayed onto it by a sprayer schematically indicatedat 14. The FR solution, with a binder, is supplied from a tank notshown, through a pump not shown.

[0033] The paper base 10, which is now coated with a layer of FRsolution, passes through IR dryers 15, while being supported by generalsupport 16, and the dryers are so adjusted as to bring the temperatureof the paper sheet to 60-70° C. The dried base paper then passes throughcoolers 17, where it is cooled by air blowers substantially to roomtemperature.

[0034] The base paper 10, carrying now a dried layer of FR compound orcompounds, including the binder, is indicated by numeral 10′. Coating isapplied thereto from a bath 18 through a roller 19, which draws from thebath an aqueous phase comprising pigment and binder transfers it to thebase paper. The coated paper is then guided by roller 20. Numeral 21indicates a blade which scrapes the surface of the paper sheet, carryingthe coating color, to control the thickness of said color. Otherthickness controlling means could be used. The paper sheet is thenguided by roller 22 and passes through dryers 23, which are similar todryers 15, though they may deliver a different amount of heat. Thedrying is carried out at this stage, as well as in the previous stage,in such a way as to bring the paper sheet to temperatures from 20 to 90°C. and preferably not lower than 60° C. The dried paper then passesthrough two coolers 24, similar to coolers 17, but that are socontrolled, in combination with the other parameters of the process andparticularly the traveling speed of the paper web, as to heat the paper,in the drying phase, to temperatures between 20° C. and 90° C., butpreferably not lower than 60° C. The weight of the coating is from 5 to30 g/m², preferably 10-15 g/m². The coated and dried paper is the finalpaper product, which is indicated at 10″. Paper sheet 10″ is wound up onthe roller system 25, which has also the task of applying to the papersheet sufficient traction to draw it through various stages of theapparatus described.

[0035] The sprayer 14 is not illustrated, since it may any conventionalsprayer. In its simplest form, it may consists of a pipe, preferably ofplastic, provided with nozzles, which are spaced at such distances as tocoat the whole width of the base paper with a uniform layer of the FRcompounds and the binder.

[0036] As has been said, a preferred FR compound is ammonium bromide andis applied to the base paper, preferably by spraying, in amounts between0.01% and 10% of the weight of the paper. It is applied to the paper asan aqueous solution having a concentration between 10 wt % and 80 wt %,but preferably between 20 wt % and 40 wt %.

[0037] The coating may be any conventional coating. For example, it maybe prepared by providing a pigment slurry and adding to it a binder,preferably starch or e.g. a latex, and mixing well, e.g. for 45 minutesbefore using. The thickness of the coating layer, when wet, on the papermay be, for example, from 5 μm to 30 μm. The coated paper is dried indrying unite that are preferably IR dryers, which are so controlled, incombination with the other parameters of the process and particularlythe traveling speed of the paper web, as to heat the paper, in thedrying phase, to temperatures between 20° C. and 90° C., but preferablynot lower than 60° C. The weight of the coating is from 5 to 30 g/m²,preferably 10-15 g/m². Preferably, the paper carrying the FR compound orcompounds is dried, before applying the coating, in the same way inwhich the coated paper is dried and in similar drying units.

[0038] The sprayed paper is coated with a coating color consisting, forexample, of the following formula (Table I): TABLE I Solids content  62%pH  8.3 pph (parts per 100 parts of dry pigments) Pigment 100 Binder  10Water-soluble polymer  1

[0039] The coating color adding to it a binder,latex or starch andmixing, the pigment slurry and adjusting the pH to the right level. Thewater-soluble polymer is added with the required water addition toachieve the right solids content.

[0040] Ignition and non-ignition tests are carried out as describedhereinafter.

[0041] The amount of starch, when the binder is starch as preferred, inthe solution applied to the paper base, is from 0.5 to 20 wt %,preferably 4-8 wt %.

EXAMPLE

[0042] The base paper was chemical pulp base paper with a weight of 75g/m². The base paper was sprayed with a solution containing 38 wt % ofammonium bromide and 6 wt % of starch. The speed of travel of the basepaper was 100 m/s. The average weight of FR compound and starch on thebase paper was 5.5 g/m². The mean tear index of the base paper was 9.13mNn²/g. The mean tensile index was 53.6 Nm/g. If said base paper iscoated in a conventional way, without spraying FR compounds, to anamount of about 8 g/m², the mean tear index is lowered to 8.05 mNn²/gand the mean tensile index to 48.0 Nm/g. After spraying the FR compoundand the starch together, as stated hereinbefore, the mean tear index is8.51 and the mean tensile index is 47.0; but if coating is later appliedto the paper, the mean tear index is lowered to 7.45 and the meantensile index to 41.2. It is seen that both those indices are lowered bythe application of FR compound, but not to a considerable extent, whiletheir ratio is lowered only from 5.95 to 5.53.

[0043] The mechanical properties of the paper resulting from the Exampleare set forth more completely in the following Tables II and III. TABLEII Tensile Index Nm/g coated paper % 18 0 16 2.5 14.9 5 14 10

[0044] TABLE III Tear index mNm²/g coated paper % 5.1 0 5.7 2.5 5.2 55.3 10

[0045] The effects of the flame-retardation were determined as set forthhereinafter.

Test Method SF5-EN ISO 6940 Textile Fabrics—Burning BehaviorDetermination of Ease of Ignition of Vertically Oriented Specimens

[0046] Samples

[0047] Sufficient number of specimens are tested until at least fivecases of ignition and five cases of non-ignition occur. Size of thespecimens is 80 mm×80 mm or 200 mm×80 mm. Specimens are conditioned at atemperature of 20±2° C. and a relative humidity of 65±2%.

[0048] Procedure, Surface Ignition

[0049] Specimen is placed on the pins of the specimen holder so that thespecimen is at least 20 mm removed from the frame. The specimen holderis placed vertically in the combustion chamber where a methane gas flamewith length of 40 mm is directed to the surface of the specimen. Thedistance between the gas nozzle and the surface of the specimen is 17mm. The flame is applied to the specimen of (size 80×80 mm) for a timeperiod which preliminary testing indicated will approximate to theminimum ignition time. Ignition shall be deemed to have occurred ifeither the flame on the specimen persists for a period of at least 5seconds after removal of the igniting flame, or if the specimen burns tothe top of the vertical edges of the specimen, after removal of ignitingflame. If the specimen burns to the top edge of the specimen duringapplication of the igniting flame, the sample is retested using a 200×80mm specimen. Ignition of the specimen is recorded. If ignition hasoccurred then the timer setting is reduced by 1 second. If ignition hasnot occurred, the timer setting is increased by up to a maximum time of20 seconds. Each trial is repeated on a new specimen continuing testinguntil there are at least five cases of ignition and five cases ofnon-ignition.

[0050] The mean of recorded times at which ignition or non-ignition,whichever has occurred least, was observed is calculated for eachdirection. If the calculated times relate to ignition, 0.5 is subtractedfrom the mean value calculated. If the calculated times relate tonon-ignition, 0.5 is added to the mean value. The resulting value isrounded to the nearest second.

[0051] Flammability Tests

[0052] The flammability tests were performed for the sprayed paper. Theresults show that the addition of ammonium bromide affects theflammability point of the paper. The results in Table IV reveal that 2%and 3.3% ammonium bromide shifts the ignition time and it does notignite. TABLE IV Test Ignition Afterflame Afterglow Ignition/ specimentime(s) time(s) time(s) non-ignition Sample: White paper, 0.7% 1 1 2 48Non-ignition 2 1 2 43 Non-ignition 3 2 2 42 Non-ignition 4 3 1 45Non-ignition 5 5 1 58 Non-ignition 6 10 0 30 Non-ignition 7 20 0 20Non-ignition Sample: White paper, 2.0% 1 1 0 7 Non-ignition 2 20 0 0Non-ignition Sample: White paper, 3.3% 1 1 0 0 Non-ignition 2 20 0 0Non-ignition 3 20 0 0 Non-ignition

Paper Tests

[0053] The paper tests show that the basic properties of the coatedpaper (folding endurance, tearing strength, tensile strength) changes ina way that the tensile strength decreases and tearing strengthincreases. The changes shown in Tables II and III imply that tensilestrength would decrease and the tearing strength would increase, whenammonium bromide is sprayed between the paper and the coating. However,the ratio between them does not vary appreciably.

[0054] The minimum ignition time for the paper used in the Example,without flame-proofing compounds, was 1 second. The flame-proofed paperdid not ignite after 20 seconds of ignition time. After shorter ignitiontime, the afterglow time was longer than the afterglow time for a longerignition time.

[0055] The longer ignition time burns a hole through the paper but thedamage is not spread.

[0056] While embodiments of the invention has been described by way ofillustration, the invention may be carried into practice with manymodifications, variations and adaptations, without exceeding the scopeof the claims.

1. A flame-retarded paper product, comprising as a fire-retardant (FR)ingredient a compound selected from among inorganic and organicflame-retardant compounds, particularly ammonium bromide and itsmixtures with other ammonium salts, mono- or diammonium phosphate,ammonium polyphosphate, ammonium sulphate, urea, organic brominatedflame retardants and their mixtures.
 2. A flame-retarded paper productaccording to claim 1 wherein the FR ingredient is ammonium bromide.
 3. Aflame-retarded paper product according to claim 1 or 2 wherein theconcentration of the ammonium bromide solution is between 10 w/w % to 90w/w %.
 4. A flame-retarded paper product according to any one of claims1 to 3 comprising a flame-retardant or flame-retardant mixture in anamount of between 0.01% by weight of the paper to about 10% by weight ofthe paper, on a dry basis.
 5. A flame-retarded paper product accordingto claim 4 wherein the flame-retardant is present in an amount ofbetween 1.5% by weight of the paper to about 4% by weight of the paper,on a dry basis.
 6. A flame-retarded paper product according to claim 1,which comprises a base paper, a layer of an FR compound or compounds,preferably including a first binder, and an uppermost layer of a coatingcolor.
 7. A flame-retarded paper product according to claim 6, whereinthe first binder is starch.
 8. A flame-retarded paper product accordingto claim 6, wherein the coating color layer comprises a second binder.9. Method of making a flame-retarded paper product, which methodcomprises: a) producing a base paper; b) applying to the paper base atleast one FR compound, to form a layer comprising said compound; and c)applying to the base paper, over said layer, a coating color and/orlaminate and/or surface sizing.
 10. Method of making a flame-retardedpaper product according to claim 9, which comprises applying the atleast one FR compound preferably by on the base paper a liquid phasecomprising the FR compound and a first binder.
 11. Method of making aflame-retarded paper product according to claim 10, wherein the firstbinder is starch.
 12. Method of making a flame-retarded paper productaccording to claim 9, further comprising drying the FR compound layerbefore applying the coating color.
 13. Method of making a flame-retardedpaper product according to claim 9, wherein the coating color comprisesa second binder and a pigment or pigments, and is applied by passing thepaper with the FR compound layer through a bath or by spreading it ontosaid paper.
 14. Method of making a flame-retarded paper productaccording to claim 9, comprising controlling the thickness of thecoating color layer.
 15. Method of making a flame-retarded paper productaccording to claim 9, wherein after drying the FR compound layer islocated between the base paper and the coating color layer, and saidcoating color layer has a thickness of about 5 to about 30 μm. 16.Method of making a flame-retarded paper product according to claim 9,wherein the flame retardant is provided in an aqueous solution. 17.Method of making a flame-retarded paper product according to claim 16,wherein the flame retardant aqueous solution is a solution of ammoniumbromide having a concentration from 10 w/w % to 90 w/w %.
 18. Method ofmaking a flame-retarded paper product according to claim 17, wherein theconcentration of the ammonium bromide solution is between 20 w/w % toabout 40 w/w %.
 19. Method of making a flame-retarded paper productaccording to claim 9, wherein the quantity of the FR compound sprayedonto the base paper is from 0.01% by weight of the paper to 10% byweight of the paper.
 20. Method of making a flame-retarded paper productaccording to claim 12, wherein the paper is dried at temperaturesbetween 20° C. to about 90° C.
 21. Use of ammonium bromide as aflame-retarding agent coating layer for paper.
 22. Improvement to papercoating machines, which comprises adding a spraying device provided withnozzles to said machines before the coating unit.
 23. Improvement topaper coating machines, which comprises adding a dispersion-applyingdevice to said machines before the coating unit.
 24. Improvement topaper coating machines according to claim 15, further comprising addinga dryer between the spraying device and the coating unit.